Tell me about ACID. Especially about transaction isolation levels. Which level did you use?

ACID is a set of properties of database transactions:

• Atomicity: a transaction is either fully completed or not executed at all.
• Consistency: a transaction transitions the database from one valid state to another.
• Isolation: parallel transactions do not affect each other.
• Durability: after a transaction is committed, its changes are saved even in case of failures.

Transaction isolation levels determine the degree to which the changes made by one transaction are visible to others:

• Read Uncommitted: can read uncommitted data (dirty read).
• Read Committed: only committed data can be read, preventing dirty reads.
• Repeatable Read: guarantees that repeated reads of the same data within a transaction will yield the same result, preventing non-repeatable reads.
• Serializable: the strictest level, transactions are executed as if they were sequential, preventing phantom reads.

In practice, the Read Committed or Repeatable Read levels are often used, balancing performance and data integrity. For example, PostgreSQL defaults to Repeatable Read, while Oracle uses Read Committed.